Wireless communication receiver having an adc with a limited dynamic range

ABSTRACT

The present invention provides a low cost receiver by reducing the required dynamic range of the ADC in a wireless communication receiver, without degrading the receiver performance. In the wireless communication receiver of the invention, a digital filter is used to filter digital signals from the ADC to attenuate residual interferers in the digital signals by a predetermined amount (e.g., more than that prescribed in a technical specification). This allows relaxation of tolerable quantization noise generated by the ADC to a pre-defined level to thereby substantially reduce a dynamic range of the ADC. This pre-defined level of quantization noise is higher than a level prescribed by the receiver&#39;s sensitivity, while the total interference of the receiver is kept at a level not greater than an allowable level. Thus, the ADC has a word length that corresponds to the reduced dynamic range. Accordingly, not only the cost of the ADC is decreased, the costs of all signals processing modules following the ADC are also decreased, resulting in a substantial reduction in the overall cost of the receiver.

BACKGROUND OF THE INVENTION

The invention relates generally to wireless communication devices, andmore particularly to wireless communication receivers, such as radiofrequency (RF) receivers.

Wireless communications technology has brought much convenience topeople's life. For example, cellular phones have been very popular andwidely used by many people. As the number of wireless equipmentincreases, mutual interferences between different equipment is of anincreasing concern with respect to both the system configurations andthe designs of wireless equipment. In designing wireless equipment, itis necessary to have the receiver attenuate all possible interferers toa sufficiently low level in order to achieve a required signalto-interference ratio (SIR) for quality communications.

FIG. 1 shows a conventional RF receiver 10, which includes a processingunit 15, an n-bit analog-to-digital converter (ADC) 52, and ademodulator 62. In processing unit 15, signals received by an antenna 11are filtered by a RF band-pass filter 12 to ensure that only the wantedsignal passes through while the strong interferers far away on thefrequency band from the wanted signal are attenuated. A low noiseamplifier (LNA) 16 amplifies the weak signal received. Then mixers 22and 36 convert the wanted signal from RF to baseband by mixing it withfrequency signals f₁ and f₂ respectively. An intermediate frequency (IF)filter 32 also attenuates the out-of-band interferers to some extent. Atbaseband, an analog low-pass filter 42 removes most of the out-of-bandinterferers and noise power to increase the SIR. An automatic gaincontrol (AGC) unit 46 adjusts its input signal into a limited dynamicrange (DR), so that ADC 52 with a limited word length can be used toconvert the analog signal into a digital signal. Thereafter, ademodulator 62 de-spreads and decodes the digital signal to recover thetransmitted user data.

In order to achieve the required SIR, the interference (I) componentshould be maintained within a tolerable range. The interference at theinput of demodulator 62 mainly comprises the residual externalinterferers and the receiver noise, which includes the circuit noisefrom all the components in the receiver and the ADC quantization noisegenerated during the sampling operation. The circuit noise remainssubstantially constant, while the ADC quantization noise is specified bythe receivers sensitivity and usually contributes little to the overallreceiver noise.

An Important feature of the ADC is its word length which specifies thenumber of bits for each sampling of the input signal. The word lengthdepends on the dynamic range requirement of the ADC. The lower limit ofthe dynamic range is specified by the equivalent quantization noiselevel as prescribed by the receiver sensitivity and the required SIR,while the upper limit is specified by the equivalent peak power of theADC input. In a receiver in which the out-of-band interferer is notsufficiently attenuated by the analog filters, the residual interfereralso has influence on the peak power at the ADC input. In some cases,the residual interferer may be much stronger than the wanted signal andreceiver noise, and therefore its power level specifies the equivalentpeak power of the ADC input. In such cases, the dynamic range requiredfor the ADC is greatly increased, since the specified equivalentquantization noise remains at a very low level. This causes ansubstantial increase to the overall cost of the receiver, since not onlythe cost of the ADC is increased as a result of an increase in its wordlength, the costs of all the signal processing modules following the ADC(e.g., the demodulator) have to be increased to accommodate thecomplexities in handling the resulting larger digital data output fromthe ADC.

FIG. 2 illustrates an example in connection with a conventionalreceiver, with reference to the TD-SCDMA specification. In this example,the equivalent receiver noise is −104.15 dBm and the specifiedequivalent quantization noise is at −119.15 dBm, which is much lowerthan the overall receiver noise. The maximum specified power level of anadjacent channel interferer is −54 dBm and is suppressed by the analogfilters to −76 dBm. This residual interferer may be further suppressedby a digital filter to −87.24 dBm. Taking into consideration of a knownpeak-to-average ratio of 12 dB, the peak power of the equivalent the ADCinput is −64 dBm. Therefore, the required dynamic range of the ADC Isthe difference between −64 dBm and −119.15 dBm, i.e., 55.15 dB. Thisusually translates to an equivalent word length between 8 to 10 bitlong. As described above, the longer the ADC's word length is, thehigher the overall receiver's cost is.

Therefore, there is a need to provide a low cost receiver withoutdegrading the performance.

SUMMARY OF THE INVENTION

The present invention provides a low cost receiver by reducing therequired dynamic range of the ADC in a wireless communication receiver,without degrading the receiver performance.

According to one embodiment of the invention, a wireless communicationreceiver is provided. The receiver comprises a processing unit, ananalog-to-digital converter (ADC), and a digital filter. The processingunit processes received signals and filters the processed signals in ananalog domain to output filtered analog signals. The ADC converts thefiltered analog signals into digital signals. Then, the digital filterfilters the digital signals from the ADC and attenuates residualinterferers in the digital signals by a predetermined amount (e.g., morethan that prescribed in a technical specification). This allowsrelaxation of tolerable quantization noise generated by the ADC to apre-defined level to thereby substantially reduce a dynamic range of theADC. This pre-defined level of quantization noise is higher than a levelprescribed by the receivers sensitivity, while the total interference ofthe receiver is kept at a level not greater than an allowable level.Thus, the ADC has a word length that corresponds to the reduced dynamicrange.

Accordingly, not only the cost of the ADC is decreased, the costs of allthe signal processing modules following the ADC are also decreased,resulting in a substantial reduction in the overall cost of thereceiver.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example,with reference to the accompanying drawings wherein:

FIG. 1 shows a conventional RF receiver;

FIG. 2 illustrates an example in connection with a conventionalreceiver;

FIG. 3 shows a wireless communication receiver according to oneembodiment of the invention;

FIG. 4 illustrates an example of reducing the dynamic range of an ADCfor a handset receiver in accordance with one embodiment of theinvention; and

FIG. 5 illustrates a transfer function of a digital filter In accordancewith one embodiment of the invention.

Throughout the drawings, the same reference numerals indicate similar orcorresponding features or functions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a wireless communication receiver 80 according to oneembodiment of the invention. Receiver 80 includes a processing unit 15,an m-bit ADC 84, a digital low-pass filter 86, and a demodulator 62.Processing unit 15 performs a number of functions including mixing andfiltering signals in the same manner as previously described inconnection with FIG. 1. Digital low-pass filter 86 further attenuatesthe out-of-band interferers to a level lower than that prescribed In thetechnical specifications. This allows relaxation of the equivalentquantization noise level of the ADC to a higher level than thatprescribed by the receiver sensitivity without changing the receiver'sSIR. Thus, a much smaller dynamic range is achieved for ADC 84,resulting in a substantial decrease in the word length of ADC 84, andthus the overall cost of the receiver.

FIG. 4 illustrates an example of reducing the dynamic range of an ADCfor a handset receiver, such as receiver 80, in accordance with oneembodiment of the invention. Compared to the example In FIG. 2, thisexample uses similar data prescribed in the TD-SCDMA specification. Inthe example, the residual adjacent interferer is suppressed in thedigital domain by 14.24 dB, which is 3 dB more than that in FIG. 2.Since the total allowable interference (I) (including both the residualinterference and the receiver noise) is to remain constant and theresidual interference has been further decreased, the total receivernoise may be relaxed to a higher level. As the front-end noise and theADC circuit noise in the total receiver noise are almost constant underthe normal circumstances, the ADC quantization noise, which is normallyat a very low level, can thus be significantly relaxed to a higherlevel. Thus, the permissible quantization noise of the ADC is greatlyrelaxed to −90.24 dBm, maintaining the overall SIR at a constant level.Of course, in an actual implementation, the equivalent quantizationnoise level can be lower than the tolerable level of −90.24 dBm. As aresult, the required dynamic range for the ADC is reduced to 26.24 dB,i.e., the difference between −64 dBm and −90.24 dBm, which issignificantly lower than 55.15 dB in the example of FIG. 2. Therefore,the corresponding word length of the ADC can be reduced by 5quantization bits compared to that illustrated in FIG. 2, resulting in asubstantial decrease in the costs of all the signal processing modulesfollowing the ADC and thus the overall cost of the receiver.

FIG. 5 illustrates the transfer function of digital filter 86 inaccordance with one embodiment of the invention.

In the above, the invention has been illustrated in connection with a RFreceiver in a mobile terminal. The invention can also be used in areceiver of other wireless communication systems, e.g., a base station,a digital TV receiver, etc.

While the invention has been described in conjunction with specificembodiments, it is evident that many alternatives, modifications andvariations will be apparent to those skilled in the art in light of theforegoing description. Accordingly, it is intended to embrace all suchalternatives, modifications and variations as fall within the spirit andscope of the appended claims.

1. A wireless communication receiver, comprising: (a) a processing unitthat processes received signals and filters the processed signals in ananalog domain to output filtered analog signals; (b) ananalog-to-digital converter (ADC) that converts the filtered analogsignals into digital signals; and (c) a digital filter that filters thedigital signals from the ADC and attenuates residual interferers in thedigital signals by a predetermined amount, so as to allow relaxation oftolerable quantization noise generated by the ADC to a pre-defined levelto thereby substantially reduce a dynamic range of the ADC; wherein theADC has a word length corresponding to the reduced dynamic range.
 2. Thereceiver of claim 1, wherein the pre-defined level is higher than alevel prescribed by the receiver's sensitivity.
 3. The receiver of claim1, wherein the pre-defined level of the quantization noise is maintainedwithin a range, such that the total interference of the receiver is keptat a level not greater than an allowable level.
 4. The receiver of claim1, further comprising a demodulator that demodulates the filtereddigital signals from the ADC to recover user data.
 5. A method for usein a wireless communication receiver, the method comprising the stepsof: processing received signals; filtering the processed signals in ananalog domain to output filtered analog signals; converting the filteredanalog signals into digital signals; and filtering the digital signalsin a digital domain to attenuate residual interferers in the digitalsignals by a predetermined amount, so as to allow relaxation oftolerable quantization noise generated at the converting step to apre-defined level to thereby substantially reducing the number ofquantization bits required at the converting step; wherein theconverting step converts the filtered analog signals into the digitalsignals with a corresponding reduced number of quantization bits.
 6. Themethod of claim 5, wherein the pre-defined level is higher than a levelprescribed by the receiver's sensitivity.
 7. The method of claim 5,wherein the pre-defined level is maintained within a range, such thatthe total interference of the receiver is kept at a level not greaterthan an allowable level.
 8. The method of claim 5, further comprising astep of demodulating the filtered digital signals to recover user data.